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Ultimate GMT400 LS Swap Guide

by Frank
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GMT400 LS Swap: The Ultimate Guide to Modernizing Your Classic Pickup

The GMT400 platform holds a special place in the hearts of truck enthusiasts. Known for their rugged looks and dependability, these trucks have become a staple among classic truck fans. However, many owners are looking to breathe new life into their GMT400 by performing a GMT400 LS swap. This guide will provide a detailed overview of what it takes to modernize your classic pickup by swapping in an LS engine.

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Understanding the GMT400 Platform

gmt400 ls swapBrief History of the GMT400

The GMT400 platform was introduced by General Motors in 1988 and spanned through 1999. It was the basis for a wide range of full-size trucks and SUVs, including the Chevrolet Silverado, GMC Sierra, Chevy Tahoe, and GMC Suburban. The GMT400 was a major redesign of GM’s truck line, focusing on improved aerodynamics, fuel efficiency, and ride quality. These trucks quickly became popular for their combination of comfort and capability.

The Platform’s Impact on the Pickup Truck Market

The GMT400 series played a crucial role in shaping the pickup truck market, as it set the stage for modern full-size trucks. Its design philosophy prioritized comfort and utility, making these trucks appealing to a broader range of consumers. Today, the GMT400 is considered a classic, and many enthusiasts are restoring or modifying these trucks to fit their specific needs and tastes.

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Reasons Why GMT400 Trucks Are Ideal Candidates for LS Swaps

One of the most popular modifications for GMT400 trucks is the LS swap. There are several reasons why GMT400 trucks are perfect candidates for this upgrade:

  1. Availability: With millions of these trucks produced, there is no shortage of potential donor vehicles for a GMT400 LS swap.
  2. Compatibility: The LS engine family shares many similarities with the original engines found in GMT400 trucks, making the swap relatively straightforward.
  3. Performance and Efficiency: Swapping an LS engine into a GMT400 truck can greatly improve power, fuel efficiency, and overall performance.

Introduction to the LS Engine Family

gmt400 ls swap familyOverview of LS Engines and Their Role in GM Vehicles

The LS engine family is a series of small-block V8 engines produced by General Motors since 1997. These engines were designed as a replacement for the aging Gen I and Gen II small-block engines. LS engines have become the gold standard for performance and reliability, thanks to their compact design, lightweight aluminum construction, and modern technology.

Advantages of LS Engines Over Older Generations

LS engines offer several advantages over the older generations of small-block engines found in GMT400 trucks:

  1. Increased power: LS engines produce significantly more horsepower and torque than their predecessors.
  2. Improved fuel efficiency: The modern design of LS engines allows for better fuel economy, even with increased performance.
  3. Enhanced reliability: The use of advanced materials and engineering techniques make LS engines more reliable and longer-lasting than their predecessors.
  4. Extensive aftermarket support: The popularity of LS engines has led to a vast array of aftermarket parts and support, making it easy to find performance upgrades and swap components.

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Popular LS Engine Models for Swaps

There are several popular LS engine models that enthusiasts often use for GMT400 LS swaps:

  1. LS1: Introduced in the 1997 Chevrolet Corvette, the 5.7L LS1 is the first engine in the LS family and offers a substantial power upgrade over stock GMT400 engines.
  2. LS2: The 6.0L LS2 debuted in the 2005 Corvette and offers even more power and torque than the LS1.
  3. LS3: Found in the 2008 Corvette and later Camaros, the 6.2L LS3 is a popular choice for those seeking maximum power from a naturally aspirated engine.
  4. LQ4/LQ9: These 6.0L iron-block engines were used in various GM trucks and SUVs, making them a budget-friendly option with great potential for performance upgrades
  5. LM7/L59: The 5.3L LM7 and L59 engines are commonly found in Silverado’s and Sierras from the early 2000s, offering a balance of performance, affordability, and availability.

Preparing for the GMT400 LS Swap

gmt400 ls swap sourceAssessing Your Current GMT400 Truck

  1. Identifying potential issues: Before starting the GMT400 LS swap, it’s essential to inspect your truck for any existing issues, such as rust, suspension problems, or electrical concerns. Addressing these issues before the swap will help ensure a smoother process and better results.
  2. Making necessary repairs before the swap: Once you have identified any problems, it’s crucial to make the necessary repairs or upgrades to avoid complications during the swap.

Selecting the Right LS Engine for Your Swap

  1. Factors to consider: When choosing an LS engine for your GMT400 LS swap, you’ll need to consider your power goals, budget, and intended use (street, off-road, or racing). Each LS engine model has its pros and cons, so it’s essential to carefully weigh your options.
  2. Matching the engine with your goals: For example, if you’re looking for a budget-friendly option with good power potential, an LQ4 or LQ9 might be the right choice. If maximum power is your goal, an LS3 or a supercharged LSA may be more suitable.

Sourcing an LS engine

  1. Junkyard finds: Many LS engines can be found in junkyards or salvage yards, often at a fraction of the cost of a new crate engine. Be prepared to inspect the engine thoroughly and potentially invest in some repairs or upgrades.
  2. Crate engines: GM Performance and other companies offer brand-new crate engines, complete with warranties and ready for installation. While more expensive, this option ensures you have a fresh, reliable engine.
  3. Remanufactured engines: Another option is to purchase a remanufactured LS engine from a reputable supplier. These engines are typically rebuilt to meet or exceed factory specifications and can offer a cost-effective and reliable alternative to a new crate engine.

Required GMT400 LS Swap Components

Engine Mounts and Adapters

To install the LS engine in your GMT400 truck, you’ll need engine mounts or adapter plates designed for the GMT400 LS swap. Several companies offer bolt-in solutions that make the installation process straightforward and secure.

Transmission and Drivetrain Considerations

  1. Adapting your existing transmission: If you choose to retain your truck’s existing transmission, you’ll need an adapter to mate it to the LS engine. Depending on your transmission model, you may also need a new torque converter and flexplate.
  2. Upgrading to a newer transmission: Many enthusiasts opt to upgrade to a modern GM transmission, such as the 4L60E, 4L80E, or T56. These transmissions offer improved performance, efficiency, and durability compared to older transmissions.

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Cooling System Upgrades

An LS engine will typically require a more substantial cooling system than the stock GMT400 setup. You’ll need a larger radiator, new hoses, and potentially a new water pump and thermostat to ensure adequate cooling for your new engine.

Fuel System Modifications

To supply the LS engine with the necessary fuel, you’ll need to upgrade your truck’s fuel system. This may involve installing a new fuel pump, regulator, and fuel lines, as well as modifying or replacing the fuel tank to accommodate a return-style fuel system.

Exhaust System Integration

Integrating the LS engine’s exhaust system with your GMT400 truck will require new headers or manifolds, as well as custom exhaust piping. Several companies offer headers designed specifically for GMT400 LS swaps, which can simplify the installation process.

Wiring and Electronics

  1. Standalone wiring harnesses: To connect your LS engine’s electronics to your GMT400 truck, you’ll need a standalone wiring harness specifically designed for this swap. These harnesses are available from several aftermarket suppliers and will typically include all necessary connectors and instructions.
  2. Integrating with existing electronics: In some cases, you may need to integrate the LS engine’s electronics with your truck’s existing systems, such as the instrument cluster, HVAC controls, and transmission controls. This process may require some custom wiring and programming but is generally manageable for experienced DIY enthusiasts or professional shops.

Accessory Drive and Serpentine Belt Setup

When swapping an LS engine into your GMT400 truck, you’ll need to ensure that the accessory drive components (such as the alternator, power steering pump, and air conditioning compressor) are compatible with your truck’s existing systems. This may require using a specific accessory drive kit or modifying the stock components to fit your new engine.

Performing the GMT400 LS Swap

Removal of the Old Engine

Before installing the LS engine, you’ll need to remove the existing engine from your truck. This process typically involves disconnecting all electrical connections, hoses, and linkages, then using an engine hoist to lift the engine out of the engine bay.

Prepping the Engine Bay for the New Engine

With the old engine removed, it’s essential to clean and prep the engine bay before installing the new LS engine. This may involve cleaning and painting the engine bay, as well as addressing any rust or corrosion issues.

Installing the LS Engine and Transmission

Once the engine bay is prepped, you can begin the process of installing the LS engine and transmission. This will involve positioning the engine on the new mounts, bolting the transmission to the engine, and connecting any necessary accessories and components.

Connecting the New Components

With the LS engine and transmission installed, you’ll need to connect the new components to your truck’s existing systems. This may involve routing new hoses and lines, connecting the wiring harness, and installing the exhaust system.

Updating the Engine Management System

After all the components are connected, you’ll need to update the engine management system to work with your new LS engine. This may involve programming a new ECU, updating the tune, or installing a custom tuning solution.

Test-Starting and Initial Break-In

Once everything is connected and programmed, you can test-start your truck and begin the initial break-in process. This typically involves checking for leaks, adjusting timing, and monitoring temperatures and pressures to ensure everything is functioning correctly.

Post-Swap Considerations

Post swapTuning and Optimizing the LS Engine for Your GMT400

After the initial break-in, you’ll want to fine-tune your LS engine for optimal performance in your GMT400 truck. This may involve adjusting the air/fuel ratio, ignition timing, and transmission shift points to achieve the desired balance of power, drivability, and fuel efficiency.

Potential Upgrades and Modifications for GMT400 LS Swap

  1. Performance parts: With the LS engine in place, you may want to consider additional performance upgrades, such as camshafts, cylinder heads, intake manifolds, or forced induction systems.
  2. Suspension and braking improvements: Upgrading your truck’s suspension and braking systems can help accommodate the increased power and performance of your new LS engine. This may involve installing performance shocks, springs, or sway bars, as well as upgrading to larger brakes and rotors.

Maintenance and Care for Your Swapped GMT400 LS Swap

With your GMT400 LS swap complete, it’s important to stay on top of maintenance to ensure the longevity and reliability of your new engine. Regular oil changes, spark plug replacement, and coolant flushes are essential for keeping your LS engine running smoothly. Additionally, be sure to keep an eye on your truck’s upgraded systems, such as the cooling, fuel, and exhaust systems, to address any issues before they become major problems.


A GMT400 LS swap is an excellent way to modernize your classic pickup, providing increased performance, fuel efficiency, and overall enjoyment. By following this comprehensive guide, you’ll have the knowledge and resources necessary to successfully perform your own LS swap and bring new life to your beloved Chevy GMT400 truck. As you embark on this exciting project, remember that the key to a successful swap lies in careful planning, attention to detail, and a willingness to invest the time and effort required to achieve the best results. With your new LS engine in place, you’ll soon experience the thrill of driving a modernized classic truck that’s ready to take on whatever adventures lie ahead.

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